Wire sizing

Hello.I have six 310 watt panels. I want to run them in three strings of two panels in series into a combiner box. The strings will be about ten feet apart and the combiner box about fifty feet behind my building. The wire coming out of the panels is 12 AWG and I want to use 6 AWG from the panels to the combiner box and then run 2 AWG from the combiner box to the Charge controller....and from there to the batteries. I have a 15 amp breaker for each string in the combiner box. Then a 50 amp breaker in a baby box to the Charge Controller and a 60 amp breaker from the Charge controller to the batteries. The Charge controller is an Outback flex 80 amp mppt. The batteries are four six volt Surrette's in series for 24 volt system. The size of wire is larger than need be but is it okay? I have the wire already.

Thank you in advance, Clark

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Comments

Why not put the combiner box next to the center panels and then make a single set for the long run to you charge controller?

I am assuming a lot here, but if you have 60 cell panels, and combined you would have 1860 watts, lowest voltage I've seen for a string of 2 would be around 65 volts which would give roughly 30 amps if the panels were producing 100% of name plate. That would allow for @80 feet with a 3% loss in voltage over 6 gauge wire. This would allow for the 10-15 feet to the combiner box and 60 feet to the breaker.

The drop would be less with higher voltages, and normally solar panels only produce about 75% of their name plate, so losses would be less than 3% in the real world for all but cold winter days.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.

Photowhit. That is what I am doing. They are Hanwah panel at 315 watts at 1890 watts combined. Typo on my part. The factory cables are about three feet long and are 12 AWG. I felt that going from them to the combiner box in the middle with 6 AWG and then the run into the building fifty feet with the 2 AWG. I thought that two panels in series would up my volts but the amps remain at 15 and then the combiner runs the strings in parrellel increasing the amps to 45. Is ok that I have gone to the larger size wire?

They have about a vmp of about 37 volts and IMP of 8.5, so a string of 2 would have a vmp of 74 volts and a IMP of 8.5

3 strings in parallel would have a VMP of 74 volts and an IMP of 25.5 amps. You get about 3% loss at 100 feet at 70 volts and 25.5 amps over 6 gauge wire and at 70 volts and 20 amps over 6 gauge wire about a 2.25% loss, plug in those numbers here;

You can look at the data sheet and below the Standard test conditions it has the normal operating conditions, They will normally put out @6.7 amps at 35 volt.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.

If they are going in the same location, you might go ahead and use heavier wire coming to the charge controller. No reason to wire it twice, just thought I'd suggest smaller wire, since copper cost so much.

Also remember that charge controllers are rated at output amperage, so the math looks like this for your array, 1890 watts de-rated to actual output 1890 x 75%= 1418 watts on a 24 volt system 1418watts/24volts = 59 amps or at the end of bulk you could use 19 volts for 1418/29=49 amps. So the cost effective largest array would be around 3000 watts, 3000 x .75=2250, 2250watts/29volts= @78 amps So maybe 10 panels. When checking I saw those panels had been disconnected, so you might be sure to look at 72 cell panels to match what you have.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.

Thanks again Photowhit. One question to follow up on the great info and one back to the wiring issue. I have the Outback Flex 80 MPPT Charge controller but it will only handle 2000 Watts for a 24 volt system. I think I am at the max wattage as I can't add two more 315 Hanwah in series as that would be over the max?

On the wiring issue. I have already purchased the larger gauge wire so cost isn't the issue. I wanted to make sure that if I expanded my system it would not require more of the incidentals...wire...fittings... conduit and such.Is there a way to connect the 12 AWG panel wires to the 6 AWG with exterior/water proof connections.

I could explain my ten year story, inching my way into solar from just a generator charging batteries to what I have now but probably not best on this thread or particular topic. Lots of lessons learned and money spent. More importantly, lots more to learn.

Well I looked and that is really stupid low array they recommend for an 80 amp charge controller! Ican't get the string sizing tool to load on windoz 10. A 2000 watt panel rating in most areas will NEVER produce 80 amps output. A battery bank that is 1/2 full would be at roughly 24 volts, a charge controller would want to 'present' about 15% above that voltage for best flow of current to the battery, so 24x1.15, that would be 27.6 volts, a 2000 watt array even if it was producing 2000 watts (which pretty much NEVER happens) would only produce 2000/27.6=72.5 amps. In reality the array will normally only produce about 1500 watts so the charge controller would only be putting out 54 amps.

I hope it can run near capacity, I think you can regulate the output, set the amount of output to say 72 amps and 'over panel' the controller. You can with Midnite's controller. It seems odd, perhaps someone else will chime in, if not I'll read the manual in the next couple days and figure it out.

As an example I'm running a Midnite Classic in a 24 volt system and it's about 92 amp controller at 24 volts, I think the string sizing tool will suggest up to 3200 watts of array as 'cost effective' I haven't run the numbers in years but will.

There are 2 types of wire for MP4 connectors, the connectors are only designed for 8-12 gauge wire, and 8 gauge is rare to find, the 2 types of wire used for wiring array and designed for direct sunlight are PV wire and Use2, the PV wire has very thick insulation and I don't even know if you can get it in anything heavier than 8 gauge and think I've seen that once! Use2 can be gotten in in heavier gauges and it's thin enough to fit in a connector if you really wanted to I would think you could use 6 gauge, but clip a few wires for the connector, but it's a short run to the combiner box. I would use 10 gauge or 8 gauge, if you are really want to max things out. From the combiner box to the charge controller I would use conduit just to keep things clean, in conduit you could use a higher variety of wires.

You say you have 6 gauge awg wire, Is it PV or Use2 or Rhw2 wire? Here is some info on wiring. Exposed to sun and the elements above ground, I think you are limited to these wires.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.

Also. If I we're to over panel the system 20% could I add two 250 watt panels giving me just under the 20%.Can I mix and match the 250 with the 315? Clark

If they are 72 cell panels you should be okay, you want the VMP to match closely, Bill (I think) also suggested that they be the same type of cells so if you have poly, stick with poly, if you have mono, stick with mono. I don't understand why but I respected who ever I had heard this from something about confusing the tracking.

The 80 amp MPPT pretty much can't be too bad a choice. A couple of options on future expansion, Switch to a 48 volt system would double the capacity of the charge controller, this works out okay if your expansion was also going to include a new/larger inverter.

Since these are 24 volt nominal panels, and you will have heavy wiring in place to the charge controller, you could also put them on an older style PWM charge controller, like a Morningstar TS60 (not MPPT) That sells for around $200. In this manner you could buy what ever is cheapest in your expansion and put them on the Flexmax. You would have to rewire your current panels so they were all in parallel.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.

I currently have my mono and poly panels mixed (one each in series - 2 strings) and my Morningstar MPPT is tracking without a problem, but I agree I wouldn't plan my system that way. I chose that route because the Imp of the mono and poly panels were closer to each other than the Vmp (voltage adds, current remains the same).